1. Field of the Invention
The present invention relates to a process for preparing substituted pyrazolyl chlorides by chlorinating aldehydes of the formula (II) under free-radical conditions.
2. Description of Related Art
Pyrazolyl chlorides are important units for preparing active crop protection ingredients.
Pyrazolyl chlorides are typically prepared by reacting carboxylic acids with a chlorinating agent. One advantage of this process is based on the fact that the corresponding carboxylic acids are easy to obtain and hence available on the industrial scale. This prerequisite is not satisfied in the preparation of the substituted pyrazolyl chlorides, since the corresponding carboxylic acids are not readily available.
For instance, the pyrazole-4-carboxylic acids are prepared from pyrazole-4-carboxyaldehydes (J. L. Huppatz: Aust. J. Chem. 36, 135-147 (1983)) with potassium permanganate according to scheme I. One problem in the industrial implementation of this oxidation is the formation of large amounts of manganese dioxide.

JP 7070076 describes the preparation of pyrazole-4-carboxylic acid by reaction of pyrazole-4-carbaldehyde with hydrogen peroxide and sodium chloride in an aqueous system under acidic conditions and in the presence or absence of a phase transfer catalyst. The space-time yields are low and the process is difficult from a safety technology point of view owing to hydrogen peroxide.
The pyrazolecarbonyl chloride then has to be prepared from the pyrazole-4-carboxylic acid in a second step with the aid of a chlorinating agent, for example thionyl chloride.
With regard to the disadvantages and problems outlined above, there is the need to provide a process which, in one step proceeding from substituted pyrazolecarboxaldehydes, makes the corresponding substituted carbonyl chlorides available with high yields and high selectivity.